Process  and apparatus for low-temperature air fractionation

ABSTRACT

The process and the apparatus are used for low-temperature air fractionation. Input air ( 8 ) is cooled in a main heat exchanger ( 9 ) and introduced into a single column ( 12 ) for obtaining nitrogen ( 11, 43 ). A nitrogen product stream ( 15, 16, 17 ) is removed from the upper region of the single column ( 12 ). A first residual fraction ( 18, 29 ) is removed from the lower or central region of the single column ( 12 ), re-compressed ( 30 ) and then fed to the single column ( 12 ) again ( 32 ). An oxygen-containing stream ( 36 ) is removed from the single column ( 12 ) at an intermediate point and fed to a pure oxygen column ( 38 ) ( 39 ). A pure oxygen product stream ( 41 ) in the liquid state is removed from the lower region of the pure oxygen column ( 38 ). The pure oxygen product stream ( 41, 56 ) is evaporated and warmed with respect to input air ( 8 ) in the main heat exchanger ( 9 ) and finally obtained as a gaseous product ( 57 ).

The invention relates to a process for low-temperature air fractionationin which input air (8) is cooled in a main heat exchanger (9) andintroduced into a single column (12) for obtaining nitrogen (11, 43), anitrogen product stream (15, 16, 17) is removed from the upper region ofthe single column (12), a first residual fraction (18, 29) is removedfrom the lower or central region of the single column (12),re-compressed (30) and then fed to the single column (12) again (32), anoxygen-containing stream (36) is removed from the single column (12) atan intermediate point and fed to a pure oxygen column (38) (39) and apure oxygen product stream (41) in a liquid state is removed from thelower region of the pure oxygen column (38).

Processes of this type, in which, in addition to the nitrogen productfrom a single column process, pure oxygen can also be obtained asproduct, are disclosed by EP 807 792 B1 and U.S. patent application Ser.No. 11/676,773 of Feb. 20, 2007, in which the oxygen product isextracted in the liquid state from the pure oxygen product stream andremoved from the process. However, this intrinsically very economicalprocess permits only relatively small product quantities of oxygen to beobtained, approximately 1 to 2% of the quantity of air. Such plants arefrequently used to supply the electronics industry with nitrogen forsemiconductor production; there, in addition to the nitrogen, quantitiesof pure oxygen which lie above the aforementioned product quantity arefrequently needed. The invention is therefore based on the object ofspecifying a process of the type mentioned at the beginning and acorresponding apparatus in which a relatively large quantity of pureoxygen product can be produced.

This object is achieved in that the pure oxygen product stream isevaporated and warmed with respect to input air in the main heatexchanger and finally obtained as a gaseous product.

This procedure is in principle known as “internal compression” in doublecolumn processes. This is used as an alternative to gaseous productcompression (external compression) if the gaseous product is to beobtained under pressure.

However, the objective of the invention is different: the productevaporation is used here primarily to recover the liquefaction coolingenergy which is contained in the pure oxygen product stream extracted inliquid form. This is because it has transpired that the limiting factorin the quantity of oxygen product is formed by the refrigerationperformance of the plant. In the case of the invention, the liquefactioncooling energy, which in the known processes is drawn off with theoxygen product, is transferred to the input air or to a partial streamof the input air in the main heat exchanger and thus remains availableto the process (apart from the usual exchange losses).

The “main heat exchanger” is preferably formed by a single heatexchanger block. In the case of larger plants, it may be expedient toimplement the main heat exchanger by means of a plurality of streamsconnected in parallel with regard to the course of temperature andformed by components separated from one another. In principle, it ispossible for the main heat exchanger or each of these streams to beformed by two or more blocks connected in series.

Here, the term “evaporation” includes pseudo evaporation undersupercritical pressure. The pressure under which the pure oxygen productstream is introduced into the main heat exchanger can therefore also lieabove the critical pressure, as can the pressure of the heat exchangemedium which is (pseudo) condensed with respect to the pure oxygenproduct stream.

If the oxygen is needed on site under an increased pressure which liesabove the operating pressure of the pure oxygen column, it is beneficialif the pure oxygen product stream is brought to an increased pressure inthe liquid state. As a result, within the scope of the invention, a warmoxygen compressor can be dispensed with or at least designed to berelatively small.

It is also beneficial if the re-compression of the first residualfraction is performed by means of a cold compressor. Here, “coldcompressor” means a compressor which is operated with an inlettemperature of less than 200 K, preferably less than 150 K, inparticular between 90 and 120 K.

In a further refinement of the invention, a second residual fraction isremoved from the lower region of the single column and depressurized ina depressurizing machine, providing work, the mechanical energy producedduring the depressurization providing work being used at least to someextent for the re-compression of the first residual fraction. Thetransfer of the mechanical energy to the re-compressor is preferablycarried out mechanically, for example via a common shaft of thedepressurizing machine and re-compressor. In particular when there-compressor is constructed as a cold compressor, preferably only someof the mechanical energy produced by the depressurizing machine istransferred to the re-compressor; the remainder goes to a warm brakingdevice, for example a braking blower, a generator or a dissipativebrake. In a further refinement of the invention, the single column has atop condenser, in which vapour from the upper region of the singlecolumn is at least partly condensed, the first residual fraction beingat least partly evaporated in the top condenser before itsre-compression and/or the second residual fraction being at least partlyevaporated in the top condenser before its depressurization providingwork.

At least some of the condensate obtained in the top condenser isdischarged to the single column as a return flow. If the two residualfractions have the same composition, they can be led jointly through thetop condenser. Preferably, however, they are led in separate passages ofthe top condenser, in particular if they have a different composition.

It is beneficial if the second residual fraction is drawn off at thebottom of the single column.

In principle, the first residual fraction can be drawn off from thesingle column together with the second, for example at the bottom (seeEP 412793 B2). In many cases, however, it is more beneficial if thefirst residual fraction has a higher nitrogen content than the secondresidual fraction. The first residual fraction is then drawn off from anintermediate point of the single column which is arranged above thebottom, in particular above the point at which the second residualfraction is removed. The two residual fractions are then evaporatedseparately in the top condenser and fed to the re-compression and,respectively, the depressurization providing work.

In addition, the invention relates to an apparatus for low-temperatureair fractionation comprising a main heat exchanger (9) for cooling inputair (8), means (11, 43) for introducing the cooled input air into asingle column (12) for obtaining nitrogen, a nitrogen product line (15,16, 17), which is connected to the upper region of the single column(12),

a first residual fraction line (18, 29, 31, 32) for the removal of afirst residual fraction from the lower or central region of the singlecolumn (12), which is connected through a re-compressor (30) andsubsequently to the single column (12) again,

means for the removal of an oxygen-containing stream (36, 39) from anintermediate point of the single column (12) and for its introductioninto a pure oxygen column (38), and having

a pure oxygen product line (41, 56) for the removal of a pure oxygenproduct stream in the liquid state from the lower region of the pureoxygen column (38), characterized in that

the pure oxygen product line (41, 56) is connected to the main heatexchanger (9), and

the apparatus has a gas product line (57) for the removal of gaseouspure oxygen product from the main heat exchanger (9).

BRIEF DESCRIPTION OF DRAWING

The invention and further details of the invention will be explained inmore detail below with reference to an exemplary embodiment illustratedschematically in the attached figure.

DETAILED DESCRIPTION OF DRAWING

Atmospheric air 1 is taken in by an air compressor via a filter 2 andcompressed there to an absolute pressure of 6 to 20 bar, preferablyabout 9 bar. After flowing through a re-cooler 4 and a water separator5, the compressed air 6 is purified in a purifying apparatus 7, whichhas a pair of containers filled with an adsorption material, preferablya molecular sieve. The purified air 8 is cooled down to about dew pointin a main heat exchanger 9 and partly liquefied. A first portion 11 ofthe cooled air 10 is introduced into a single column 12 via a throttlingvalve 51. The feeding is carried out preferably a few practical ortheoretical plates above the bottom.

The operating pressure of the single column 12 (at the top) is 6 to 20bar, preferably about 9 bar. Its top condenser is cooled with a firstresidual fraction 18 and a second residual fraction 14. The secondresidual fraction 14 is drawn off from the bottom of the single column12, the first residual fraction 18 from an intermediate point somepractical or theoretical plates above the air feed or at the same heightas the latter. Gaseous nitrogen 15, 16 is drawn off from the top of thesingle column 12 as the main product, is warmed to approximately ambienttemperature in the main heat exchanger 9 and finally drawn off via line17 as a gaseous pressurized product (PGAN). Part 53 of the condensate 52from the top condenser 13 can be obtained as a liquid nitrogen product(PLIN); the remainder 54 is discharged to the top of the single columnas a return flow.

The first residual fraction 18 is evaporated in the top condenser 13under a pressure of 2 to 9 bar, preferably about 4 bar, and flows ingaseous form via a line 29 to a cold compressor 30, in which it isre-compressed to approximately the operating pressure of the singlecolumn. The re-compressed residual fraction 31 is cooled down to columntemperature again in the main heat exchanger 9 and finally supplied tothe bottom of the single column 12 again via line 32.

The second residual fraction 14 is evaporated in the top condenser 13under a pressure of 2 to 9 bar, preferably about 4 bar, and flows ingaseous form via line 19 to the cold end of the main heat exchanger 9.From the latter, it is removed again (line 20) at an intermediatetemperature and depressurized to about 300 mbar above atmosphericpressure, providing work, in a depressurizing machine 21, which isformed as a turbo expander in the example. The depressurizing machine iscoupled mechanically to the cold compressor 30 and a braking device 22which, in the exemplary embodiment, is formed by an oil-filled brake.The depressurized second residual fraction 23 is warmed to about ambienttemperature in the main heat exchanger 9. The warm second residualfraction 24 is blown off into the atmosphere (line 25) and/or used asregenerating gas 26, 27 in the purification apparatus 7, if appropriatefollowing heating in the heating device 28.

An oxygen-containing stream 36, which is essentially free of lessvolatile contaminants, is drawn off in the liquid state from anintermediate point of the single column 12, which is arranged 5 to 25theoretical or practical plates above the air feed. If appropriate, theoxygen-containing stream 36 is supercooled in a bottom evaporator 37 ofa pure oxygen column 38 and discharged to the top of the pure oxygencolumn 38 via line 39 and throttling valve 40. The operating pressure ofthe pure oxygen column 38 (at the top) is 1.3 to 4 bar, preferably about2.5 bar.

The bottom evaporator 37 of the pure oxygen column 38 is additionallycooled by means of a second portion 42 of the cooled input air 10. Theinput air stream 42 is in this case condensed at least partly, forexample completely, and flows via line 43 to the single column 12, whereit is introduced approximately at the height of the feed of the otherinput air 11.

From the bottom of the pure oxygen column 38, a pure oxygen productstream 41 is removed in the liquid state, brought to an increasedpressure of 2 to 100 bar, preferably about 12 bar, by means of a pump55, led via line 56 to the cold end of the main heat exchanger 9,evaporated there under the increased pressure and heated toapproximately ambient temperature and finally obtained as a gaseousproduct (GOX-IC) via line 57.

The top gas 58 from the pure oxygen column 38 is mixed with thedepressurized second residual fraction 23. If appropriate, part of theinput air is led via a bypass line 59 to the inlet of the coldcompressor 30 for the purpose of preventing the latter pumping(anti-surge control).

If required, liquid oxygen can be removed from the plant as liquidproduct upstream and/or downstream of the pump 55 (not illustrated inthe drawing). In addition, an external liquid, for example liquid argon,liquid nitrogen or liquid oxygen, from a liquid tank can be evaporatedin the main heat exchanger 9 in indirect heat exchange with the inputair (not illustrated in the drawing).

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The preceding preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing, all temperatures are set forth uncorrected in degreesKelvin.

The preceding embodiment can be repeated with similar success bysubstituting the generically described operating conditions of thisinvention for those used in the embodiment.

The entire disclosures of all applications, patents and publications,cited herein and of corresponding German application No. 10 2007024168.4, filed May 24, 2007, are incorporated by reference herein.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

1. A Process for low-temperature air fractionation, in which input air(8) is cooled in a main heat exchanger (9) and introduced into a singlecolumn (12) for obtaining nitrogen (11, 43), a nitrogen product stream(15, 16, 17) is removed from the upper region of the single column (12),a first residual fraction (18, 29) is removed from the lower or centralregion of the single column (12), re-compressed (30) and then fed to thesingle column (12) again (32), an oxygen-containing stream (36) isremoved from the single column (12) at an intermediate point and fed toa pure oxygen column (38) (39) and a pure oxygen product stream (41) ina liquid state is removed from the lower region of the pure oxygencolumn (38), characterized in that the pure oxygen product stream (41,56) is evaporated and warmed with respect to input air (8) in the mainheat exchanger (9) and is finally obtained as a gaseous product (57). 2.A Process according to claim 1, characterized in that the pure oxygenproduct stream (41) prior to evaporation is brought to an increasedpressure (55) in the liquid state.
 3. A Process according to claim 1,characterized in that the re-compression (30) of the first residualfraction (18, 29) is performed in a cold compressor.
 4. A Processaccording to claim 1, characterized in that a second residual fraction(14, 19) is removed from the lower region of the single column (12) andis depressurized in a depressurizing machine (21), providing work, themechanical energy produced during the depressurization providing workbeing used at least to some extent for the re-compression of the firstresidual fraction.
 5. A Process according to claim 4, characterized inthat the single column (12) has a top condenser (13), in which vapourfrom the upper region of the single column is at least partly condensed,the first residual fraction (18) being at least partly evaporated in thetop condenser before its re-compression (30) and/or the second residualfraction (14) being at least partly evaporated in the top condenserbefore its depressurization providing work (21).
 6. A Process accordingto claim 4, characterized in that the second residual fraction (14) isdrawn off at the bottom of the single column (12).
 7. A Processaccording to claim 4, characterized in that the first residual fraction(18) is drawn off from an intermediate point of the single column (12)which is arranged above the bottom, in particular above the point atwhich the second residual fraction (14) is removed.
 8. Apparatus forlow-temperature air fractionation, comprising: a main heat exchanger (9)for cooling input air (8), means (11, 43) for introducing the cooledinput air into a single column (12) for obtaining nitrogen, a nitrogenproduct line (15, 16, 17), which is connected to the upper region of thesingle column (12), a first residual fraction line (18, 29, 31, 32) forthe removal of a first residual fraction from the lower or centralregion of the single column (12), which is connected through are-compressor (30) and subsequently to the single column (12) again,means for the removal of an oxygen-containing stream (36, 39) from anintermediate point of the single column (12) and for its introductioninto a pure oxygen column (38), and having a pure oxygen product line(41, 56) for the removal of a pure oxygen product stream in the liquidstate from the lower region of the pure oxygen column (38),characterized in that the pure oxygen product line (41, 56) is connectedto the main heat exchanger (9), and the apparatus has a gas product line(57) for the removal of gaseous pure oxygen product from the main heatexchanger (9).
 9. Apparatus according to claim 8, characterized in thatmeans (55) for increasing pressure in the liquid state are arranged inthe pure oxygen product line (41, 56).
 10. Apparatus according to claim8, characterized in that the re-compressor (30) is constructed as a coldcompressor.
 11. A Process according to claim 5, characterized in thatthe first residual fraction (18) is drawn off from an intermediate pointof the single column (12) which is arranged above the bottom, above thepoint at which the second residual fraction (14) is removed.
 12. AProcess according to claim 11, characterized in that the second residualfraction (14) is drawn off at the bottom of the single column (12).